1. Field of the Invention
The present invention relates to a heat treatment apparatus and a heat treatment method.
2. Description of the Related Art
Various heat treatment apparatuses are employed in a semiconductor device fabricating process for subjecting a semiconductor wafer, i.e., a workpiece, to oxidation, diffusion, CVD (chemical vapor deposition), annealing and such. A representative batch heat treatment apparatus among those known heat treatment apparatus has a vertical reaction tube defining a heat treatment chamber which receives wafer boat holding a plurality of wafers, for example, 150 wafers, at predetermined intervals in a vertical stack, and subjects the plurality of wafers to a heat treatment in an atmosphere of a processing gas.
This known batch heat treatment apparatus is capable of heat-treating a plurality of wafers simultaneously and is satisfactory to some extent. However, since the upper wafers held in an upper portion of the wafer boat and the lower wafers held in a lower portion of the wafer boat are spaced a relatively long distance apart, the upper wafers are brought first into a hot environment in the heat treatment chamber and are taken last out of the hot environment, and operations for loading and unloading the heat treatment chamber need time, it is hard to heat all the wafers evenly by a heat treatment in which the temperature is raised and lowered quickly.
There have been proposed single-wafer heat treatment apparatuses capable of carrying out a heat treatment method requiring rapid heating and rapid cooling suitable for processing wafers of a large diameter and for fabricating miniaturized semiconductor devices. These previously proposed single-wafer heat treatment apparatuses are classified into those of a hot wall type which heat a heat treatment chamber together with a wafer by a heating means disposed outside the heat treatment chamber, and those of a cold wall type which heat only a wafer placed in a heat treatment chamber through a susceptor disposed within the processing chamber.
However, when a wafer is heated by those previously proposed heat treatment apparatus of either the hot wall type or the cold wall type, there is a tendency that the temperature of the central portion of the wafer starts rising later than that of the peripheral portion of the wafer and, consequently, that the surface of the wafer is heated in an irregular temperature distribution while the wafer is being heated to a desired temperature. Particularly, in the heat treatment apparatus of the cold wall type, the irregularity of the temperature distribution in the surface of the wafer is increased even in a heat-treating period after the wafer has been heated to the desired temperature by the process gas of a low temperature applied to the heated wafer. Such an irregular temperature distribution in the surface of the wafer tends to deteriorate the uniformity of heat-treating, such as film formation. Moreover, the process gas adsorbed by the wafer needs time to be heated to a temperature equal to that of the wafer, so that heat treatment apparatus has a tendency to delay the reaction between the processing gas and the wafer, deteriorate the quality of the product and reduce the throughput of the process.